It is well known in the art of drilling oil and gas wells to secure drill collars within or adjacent to the bottom hole assembly of a drilling string, above the drill bit. Typically, a drill collar is a heavy, thick-walled, tubular component, used to provide weight to the drill bit such that the drill bit exerts sufficient downward force to penetrate a formation. The weight and thickness of drill collars also reduces the effect of vibrations caused by drilling on the bottom hole assembly, maintaining stability and reducing undesired directional deviation. Conventional drill collars are formed from steel bars, machined to a desired size, shape, and finish, through which a longitudinal bore is drilled to enable the flow of drilling fluid therethrough. A standard drill collar is approximately thirty feet in length, and a standard bottom hole assembly, which includes multiple drill collars, is generally three hundred feet in length.
Due to the potential for interference from ferrous metals when using logging while drilling and/or measurement while drilling devices, as well as other electrical or magnetic devices, many drill collars are formed from nonmagnetic alloys, such as monel. Often, alternative nonmagnetic materials for drill collars lack the strength of high carbide steel and rapidly become worn. Steel drill collars are also hindered by a limited usable life. Once worn, drill collars cannot normally be repaired and must be machined to a smaller diameter to enable reuse with a smaller bottom hole assembly within a smaller well bore. Drill collars worn or damaged beyond use must be discarded and replaced, significantly increasing the cost of downhole operations. To enhance wear resistance, drill collars can be provided with hardfacing, or with inserts having hardfacing thereon, the inserts being welded or otherwise attached to the drill collar. However, the limited usable life of drill collars remains a difficulty in the industry, as all drill collars, even when hardfaced, are prone to wear and damage during use.
The wear experienced by a drill collar is exacerbated by the length of the collar and correspondingly, the length of the bottom hole assembly. Due to the proximity of the outer diameter of a bottom hole assembly to the wall of a well bore, a narrow region of annular space exists along the length of the assembly, which results in a high velocity, turbulent fluid flow directly adjacent thereto. The large length required for a bottom hole assembly to provide sufficient weight to a drill bit results in prolonged exposure of the assembly to this high velocity annular fluid. The turbulent fluid flow can quickly increase wear to many components of the bottom hole assembly, including drill collars, and can also increase the potential for directional instability or undesired deviation.
Additionally, during drilling operations, it is common for drill pipe to experience lateral sway and vibrations, especially at great depths, due to the yield of the drill pipe. These vibrations are transferred to adjacent drill collars and other components of the bottom hole assembly. Vibrations are released from a drilling string at the point where the drill pipe connects to a thicker and less flexible drill collar. While drill collars generally yield less than drill pipe, conventional steel and monel drill collars are still prone to lateral movement and vibration. Due to the proximity of the bottom hole assembly to the well bore wall, under extreme conditions, conventional drill collars can be caused to yield sufficiently to contact the well bore, which can damage the drill collar or other components of bottom hole assembly or cause the bottom hole assembly to become stuck. Excessive vibration and/or yielding of the drill collars can also cause the drill bit to be lifted from the bottom of the well bore, causing ineffective and/or intermittent drilling. This difficulty is enhanced by the three hundred foot length of a conventional bottom hole assembly, as a lengthy assemblage of drill collars is more prone to yield and lateral movement than a shorter bottom hole assembly.
Further, when drilling through strata of varying pressures, such as alternating sand, shale, and/or rock layers, it is common for regions of lower formation pressure to create a vacuum, which can pull the bottom hole assembly toward the well bore wall. The bottom hole assembly is affected by such a vacuum during the duration of its passage through a region of strata having a pressure differing from that of an adjacent region of strata. Thus, the overall length of the bottom hole assembly can exacerbate the detrimental affects of differing formation pressure, increasing the likelihood of the bottom hole assembly becoming damaged or stuck.
A need exists for a drill collar that overcomes the deficiencies of conventional drill collars by resisting wear, vibration, and lateral movement and vibrations, thereby improving the useful life of the drill collar while minimizing the incidence or occurrence for an associated bottom hole assembly to impact the well bore wall or otherwise become damaged or stuck.
A need also exists for a drill collar that surpasses the limitations of conventional materials, enabling use of non ferromagnetic drill collars having a shorter length than conventional drill collars, subsequently allowing for creation of bottom hole assemblies having a significantly reduced length.